"Tomatoes may reduce breast cancer risk' was a headline that went viral recently following research that reported positive effects of eating tomatoes in women with a high risk of breast cancer.

It wasn't the first study linking brightly coloured vegetables to a lower breast cancer risk - 2012 research from Harvard Medical School also found that that having higher levels of circulating carotenoids - nutrients in red, yellow and other brightly hued fruit and veg - was also linked to a reduced risk of the disease.

With breast cancer now affecting one in nine women, headlines like these seem to offer hope - and if they get us eating more vegetables, who could argue with that?

But dietitian Kellie Bilinski has mixed feelings about media reports like these. She believes women would be better off with a louder message about the link between overweight and breast cancer risk instead.

"There's a lot of research accumulating that suggests plant based foods provide protective antioxidants which may help prevent breast cancer. But for the type of breast cancer that appears after menopause I don't think the beneficial effects of vegetables are quite as strong as the negative effect of obesity in women at this age. Eating a lot of tomatoes won't undo the damage of being overweight," says Bilinski who works with the Chris O'Brien Lifehouse cancer treatment centre in Sydney.

Headlines about certain vegetables reducing cancer risk are useful to a point. They remind us of the value of eating plant foods generally.

But there's a problem with focussing on single 'super foods' or nutrients as talismans against disease because the protective effect of plants comes from a variety of vegetables and fruit in many colours - not only reds and yellows, but greens, purples, pinks and blacks.

Even beige may have a place in this rainbow, with some research looking at potential anti-breast cancer compounds in mushrooms.

"Cancer is a complicated process that evolves step by step over time - it's not about one specific compound in food targeting a single process that causes cancer. I explain to people that the different pigments in plant foods represent different protective compounds - and that eating a diet with a broad range of these compounds is like having a toolkit that can help disarm cancer at different stages," she says.

"The other benefit of vegetables is that they're low in kilojoules so they help keep weight down and, according to the World Cancer Research Fund, 38 per cent of breast cancer can be attributed to two lifestyle factors: overweight and alcohol."

The reason extra fat increases breast cancer risk in older women is that after menopause when the ovaries stop producing oestrogen, this hormone is produced in fatty tissue -and the more fat there is the more oestrogen is produced.

"The amount of oestrogen circulating in a woman's body is in excess of what's normal and it can raise breast cancer risk," Bilinski says.

"This is why exercise is recognised as important for preventing breast cancer and breast cancer recurrence - it's not just that it helps keep weight down but because it reduces levels of oestrogen and other hormones that may contribute to cancer. One theory is that chronically high levels of insulin which can occur with overweight may be cancer promoting - and exercise helps reduce insulin."

As for the role of soyfood in preventing breast cancer, it's a question of watch this space. According to the American Cancer Society, some studies suggest soy may reduce the risk, but the evidence is inconclusive.

"The strongest evidence for a protective effect from soy comes from studies of Asian women but compared to women in western countries, they have a different pattern of eating generally, as well as different genes," Bilinski says.

"I think it may be that it's a plant-based diet that's protective, not soy alone."